This invention relates to a VTOL aircraft having a plurality of motors about the wing that are tiltable to provide a hovering capability in a level position or a steep nose down attitude. It relates in particular to an improved aircraft with large flaps for steeper descent, feathering propellers, canted engines, provision for tilt past the vertical, emergency motors usable for gust and attitude control and special landing gear with weathercock control. Full automatic and manual systems are included.
Previous art described by Quenzler has a VTOL aircraft of this class with interconnected nacelle mounted tiltmotors upon a wing that has high lift devices to aid in maintaining aerodynamic control in translational flight. The Turner description uses the same lift principle but with two tip mounted tilt rotors.
The Quenzler types have crosswise interconnected engines with extra weight and transmission losses and no emergency power system. With eight or more tiltmotors they do not need engine interconnection or an emergency power system, but have added maintenance requirements, they would be more suitable with electric power.
The nature of VTOL aircraft with two tiltmotors as described by Turner is such as to require large rotors with cyclic pitch, engine interconnecting shafts and gearboxes, involving greater rotor complexity and expense. The large rotors at the wing tips cause laterally uneven ground effect in side gusts as one wing receives an increased ground reaction while the other wing a decreased reaction. Such rotors also present a comparatively large foreign object target in flight as typified in the Bell-Boeing V22 Osprey and the smaller Bell Eagle eye UAV.
The two tiltmotor class also has less positive pitch control in hover than the four tiltmotors of the Quenzler description, and requires specialized flaps and stringent weight control for even greater expense. Power on one engine involves shaft power losses. The overall span is roughly twice that of the four tiltmotor. Roll inertia is high. High speed versions require a two speed gearbox or variable diameter rotor to reduce tipspeed. The Dornier rotor diameters are marginly less than the V22.
The Wechsler tiltrotor is similar to the Turner but has contra-rotating propellers. these also reduce the diameter slightly to lower the tipspeed at high speed, but at a great cost in complexity, weight and even further expense than in V22 examples.
The Wilmowski tandem tiltrotor with large rotors fore and aft of the fuselage has the disadvantage that in hover the fuselage is in the forward rotor downwash and in level flight the forward rotor represents a huge obstruction to forward view from the cockpit also there could be considerable drumming due to the shockwaves of the forward rotor slipstream against the cockpit and fuselage side. The rotor span is about the same as for the four rotor Quenzler types. The design is more efficient if the forward rotors could feather for cruise, with all power to the rear rotors.
The Lariviere canard with two large boom mounted propellers between the fixed flying surfaces requires a heavy and highly stressed fuselage, giving higher inertia about the lateral axis than the Turner description and less maneuvrability especially if the fuel is forward and aft of the engine. The wing has no flaps to reduce descent speed. The fat fuselage, booms and wing pylons have high drag.
The tiltwing class such as the Canadair CL84 and Ishida TW68 with two motors and the XC-142A with four motors cannot use high lift devices as are used in the class with two tiltmotors and have tail rotors to improve pitch control in translational flight, again with gearboxes, shafting, power losses, complexity and expense. All defunct, they had however, better performance than the V22 Osprey. V44 and tandem tiltwings have higher drag, mass and inertia than the hoverplane.
The Boeing Heliwing and earlier tailsitters such as the Convair contra-prop XFY-1 and jet Ryan X-13, which avoid the tiltwing mechanism, together with jet flaps, deflecting flaps and other systems have all been abandoned nevertheless. They all lacked driveway maneuvrability, especially in gusty conditions.
The Moller types have a low efficiency lift system and a high drag lifting body fuselage plus high nacelle drag and interference drag. They are designed to be roadable vehicles, are expensive and have eight motor maintenance.
The Rutherford tipjet rotorwing requires specialized gas turbine engines and for safety two of them, feeding ducted, gimballing, teetering, tip driven and variably damped rotors. A personal aircraft would be too large and extremely expensive.
The above describes the prior art upon which the hoverplane invention provides improvement, the ability to descend quickly without undesirable forward accelerations, the smaller size, light weight, steady platform and greater agility necessary to a personal aircraft able to land in driveways at lower cost with four engine safety. The hoverplane is the first practical personal aircraft.
The hoverplane was invented as the result of considering the need to increase the safety of small private airplanes in view of numerous and continuing fatal accidents after nearly 100 years of private flying in aircraft unable to xe2x80x9cstopxe2x80x9d in the air. The existing helicopter, tiltrotor and tiltwing aircraft were considered too expensive, complex or hard to handle for use as a small private aircraft.
All types of aircraft capable of maneuvers such as slow descent that would reduce accidents were considered, then cost reduction and practical usage studied. It was realized that a small four tiltmotor aircraft could have improved safety features and could slow down and land safely in bad weather or severe icing conditions.
My flying experience with Bristol Brigand aircraft equipped with large dive brakes inflated by ram air, and also with flights in Horsa gliders having very large flaps allowing steep descent and pull-out with rapid deceleration, suggested the means to reduce speed in descent.
To obtain a light, inexpensive hovering machine with the fail-safe quality of being able to hover on two motors, the four tiltmotor arrangement was selected.
Aerodynamic characteristics were explored using models having four tiltmotors. This allowed the stability to be checked and transitional behavior to be studied. The models were flown with and without an autopilot.
A photograph of the model was taken in 1996 while hovering with a motor tilt of 75 deg and the propellers substantially horizontal, when the model remained stationary with a slight turning movement to port (due to engine torque). This is believed to be the first record of an aircraft of this type in hover. The model weighed 15 kilos with two 2 kw engines forward and two 1.5 kw engines aft.
All engines turned anti-clockwise as viewed from the spinner, causing a slight torque asymmetry. Behavior in ground-effect indicated the desirability of autopilot control under this condition for the model.
Tests also showed the need to reduce the angle of attack in descent due to stalling. Any attempt to reduce the angle of attack for descent resulted in an unwanted speed increase.
Another test resulted in accidental descent into trees. A steep nose up angle was adopted with slow descent, only minimal damage was sustained to the tail only. The flight was recorded by camera. The result shows the advantage of small rotors for emergency descent in forests.
As a result of the tests a design for a manned hoverplane was made incorporating the following improvements.
Large flaps to ensure slow and steep descent with the propellers tilted horizontal to the airstream. After take-off the flaps, when unlocked, provide a tilt reducing force as speed increases and vice verse, assisting back-up manual tilt.
Four motors for hovering, able to tilt more than 90 deg to prevent stall in descent by lowering the angle of attack.
Variable pitch feathering propellers or in another embodiment, with additional cyclic pitch control to assist tilt and improve lateral flight control in hover.
Sideways propeller tilt in transition to reduce uneven propeller side loads, providing less wear and maintenance of the hub mechanism.
An emergency power system for each motor, usable under computer control for gust alleviation, to shift the center of lift upward in hover at steep angles, nose up or nose down, and to maintain propeller RPM to balance gyroscopic forces.
A low cost manual control system having linkages that cancel tilt reactions and a throttle system that simplifies manual control in hover. The small propellers allow manual tilt control.
Special autopilot systems able to maintain a stationary hovering position while allowing the machine to weathercock. This is to allow landing with a minimum of reverse airflow over the control surfaces.
A long undercarriage to provide propeller clearance, with a weathercock sensing system. The long legs can also be fitted with inflatable balloons for water landing.
Two engines, preferably the forward engines may be shut down for economical cruise or long duration search.
The combination of overtilt and large flaps confers the advantage that a compact design is possible with improved downdraft about the wing, better handling and less disturbing ground effect. The wing, nacelles, engines and flaps together as a unit can be affixed to any type aircraft whether it is a conventional design, canard or ultra compact flying wing. Detail wing configurations will of course vary.
When the hoverplane was initially designed it was not known that any others had described this four tiltmotor arrangement and a simple low cost design without cyclic pitch propellers was being compared to a sophisticated expensive fly-by-wire system with cyclic pitch and fully autonomous flight.
This machine would have a fundamental difference from the simpler model in that a full six axis inertia system would allow lateral displacements without body tilt and would be suitable for robotic missions, an improvement over the normal helicopter which is hung at the rotor hub and must tilt the rotor before making a lateral displacement and even has reactions to the high mounted tailrotor. It is believed that the hoverplane with cyclic pitch applied to create a steady platform is new.
The hoverplane includes full use of automatic systems and constant flight control to meet the future very high safety standards needed for general use. However this design can be simplified to provide a version having lower cost than any equivalent aircraft able to hover yet cruise at high speed. It would permit life and convenient travel in areas where there are no roads or too many cars.
The design with fully automatic systems serves as the basis to the preferred embodiment A simplified embodiment with manual systems, an embodiment with canard layout and a tailless embodiment are also described. A tailless example with untwisted forward rotors that stop horizontally to act as canards is given.